The bottom end of a drillstring has a bottom hole assembly (BHA). The BHA includes a drill bit and typically also sensors, control mechanisms, and associated circuitry. The sensors may measure properties of the formation and of the fluid that is contained in the formation. A BHA may also include sensors that measure the BHA's orientation and position.
The drilling operation is controlled by an operator at the surface. The drillstring is rotated at a desired rate by a rotary table, or top drive, at the surface, and the operator controls the weight-on-bit and other operating parameters of the drilling process.
Drilling fluid, or “mud”, is pumped from the surface to the drill bit by way of the drillstring. The mud serves to cool and lubricate the drill bit, and to carry the drill cuttings back to the surface. The density of the mud is carefully controlled to maintain the hydrostatic pressure in the borehole at desired levels.
In order for the operator to be aware of the measurements made by the sensors in the BHA, and for the operator to be able to control the operation of the drill bit, communication between the operator and the BHA is necessary. A downlink is a communication from the surface to tools comprising part of the drillstring, typically within the BHA. A downlink might typically command a change of parameters for a rotary steerable system, intended to modify the curvature or direction in which the hole is progressing, or the operational parameters of downhole sensing tools. Likewise, an uplink is a communication from the BHA to the surface. An uplink is typically a transmission of the data collected by the sensors in the BHA. For example, the data may provide the BHA orientation. Uplink communications are also used to confirm that a downlink command was correctly understood.
One common method of communication is called “mud pulse telemetry”. Mud pulse telemetry involves sending signals, either downlinks or uplinks, by creating pressure and/or flow rate pulses in the mud. These pulses may be detected by sensors at the receiving location. For example, in a downlink operation, a change in the flow rate of the mud being pumped down the drillstring may be detected by a sensor in the BHA. The pattern of the pulses may be detected by the sensors and interpreted as a command for the BHA.
A commonly used technique for downlinking includes timed variation of pump speed. The downhole tool either counts transitions from high speed flow to low speed flow (and vice versa) or measures the time between certain transitions.
However, pump adjustments made at the surface are not immediately detected downhole. This is a consequence not principally of wave speed, but a combination of pressure drops and fluid compliance. Further, it is usually necessary to ensure that pump speed variations do not lead to changes in surface flow rates or pressure which exceed safety limits